1. Field of the Invention
This invention relates to a method for controlling and removing solid deposits from a surface of a component of a steam generating system, wherein the solid deposits arise from impurities introduced into the steam generating system.
2. Brief Description of the Background Art
The fact that various salts are introduced as impurities into steam generating systems is well known by those skilled in the art. For example, condenser cooling water inleakage may be the greatest or most common source. Impurities may be introduced in the form of chemical additives, ion exchange media, welding residuals, grinding operations, desiccants and related construction/maintenance functions. These introduced impurities include, such as for example, but not limited to, calcium, magnesium, sodium, potassium, aluminum, chloride, silica, and sulfate, and insoluble salts such as, for example, but not limited to, calcium sulfate and aluminum silicate. These impurities are commonly reported as solids in the surface analysis of steam cycle materials.
The role and impact of introduced impurities include various debilitating corrosion mechanisms, particularly in the boiler and steam turbine systems. The most common known methods to minimize the effect of introduced impurities include exclusion and removal on ion exchange media. It is known by those skilled in the art that steam cycles are treated with alkalizing amines directed at maintaining alkalinity or pH control and reducing agents to promote passivation and minimize corrosion from impurities inherent in the steam generating system such as, for example, iron. EPRI, Workshop On Use Of Amines In Conditioning Steam/Water Circuits, "Use Of Amines In Once Through Steam Generators", authored by G. Quadri, et al., Tampa, Fla., U.S.A., September 1990, discloses the use of dimethylamine in utility fossil fueled power plant boilers in connection with the conventional manner of steam cycle alkalization, known by those skilled in the art, to reduce corrosion of ferrous materials.
The use of alkalizing amines to minimize corrosion caused by impurities inherent to the steam generating system is not generally compatible with the objective to minimize the formation of solid deposits arising from impurities introduced into the steam generating system. For example, maintenance of high alkalinity with alkalizing amines such as, for example, ammonia, limits monovalent cation such as, for example, sodium, removal on ion exchange systems such as, for example, condensate polishers. (Salem, E., "Separation Technology Requirements For Operation In The Amine Cycle With Deep Bed Condensate Polishing", from proceedings, workshop on Use Of Amines In Conditioning Steam/Water Circuits, EPRI, Tampa, Fla. September 1990). Therefore, it is often necessary to make a compromise between these two objectives. Prior to the instant invention, attempts to achieve both objectives were economically unattractive since the ion exchange system service life was shortened.
The literature clearly establishes the effect of introduced impurities on various corrosion processes or mechanisms. For example, caustic or acid forming species can preclude protective oxide stability, leading to catastrophic failure of materials, including corrosion resistant alloys. These materials are used in critical applications including steam generator tubing that provides the safety boundary in design of, such as for example, nuclear steam generators. The solid deposits that arise from impurities introduced into the steam generating system precipitate on the surfaces of the steam generating system forming scale. The accumulation of these solid deposits and scale prevents effective heat transfer, interferes with fluid flow of the water carrying system, and facilitates corrosive processes. These solid deposits formed by impurities introduced into the steam generating system is an expensive problem in many industrial applications such as, for example, the utility power industry, causing delays and shut downs for controlling and removing these solid deposits, and adding to the costs of operating and maintaining the steam plant operations. More specifically, these introduced impurities present similar issues on critical components of steam turbines and failures are common to nuclear and fossil fueled power plants. These failures include, for example, decrease in steam pressure and reduction in high pressure turbine output resulting in a decrease in energy output of the power plant, and turbine corrosion. Further, stress corrosion failures are common on turbine components and are caused, in part, by steam formed deposits including, for example, introduced impurities.
It is well known to those skilled in the art that the electrophoretic properties of steam formed solid deposits are typically not neutral. These deposits exhibit acid and base centres which give rise to catalytic properties and to selectivity for sorption of soluble ions such as, for example, Na.sup.+ and Cl.sup.-. Solids in alkaline systems usually exhibit acidity while those in acidic systems exhibit base properties. Further, some solid deposits such as, for example, alumina silicates, are known to exhibit bifunctional properties.
The acid strength and capacity of these solid deposits such as, for example, Al.sub.2 O.sub.3 and SiO.sub.2, are influenced by many factors including the molar ratio such as, for example, between alumina and silica, by the temperature at which the solid deposit is formed, by stress induced crystalline imperfections and by gamma irradiation.
It will be appreciated by those skilled in the art that the above mentioned background art does not teach or suggest the method of controlling and removing from the surface of a component of a steam generating system solid deposits formed from an impurity introduced into the steam generating system. Therefore, it will be understood by those skilled in the art that applicants have discovered unexpectedly that the method of the present invention comprising adding to an aqueous phase of a steam generating system an effective amount of at least one volatile amine having a pKa value greater than about 10.61 and providing thermal cycling results in controlling and removing solid deposits from the surface of a component of the steam generating system that are superior to results of others previously achieved.
In spite of this background material, there remains a very real and substantial need for a method of controlling and/or removing solid deposits arising from introduced impurities from the surface of a component of a steam generating system.